1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the selective upgrading of naphtha by isomerization.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher- severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Such key options as increased reforming severity and higher FCC gasoline octane result in a higher aromatics content of the gasoline pool, through the production of high- octane aromatics at the expense of low-octane heavy paraffins.
Currently, refiners are faced with the prospect of supplying reformulated gasoline to meet tightened automotive emission standards. Reformulated gasoline differs from the traditional product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics. Benzene content generally is being restricted to 1% or lower, and is limited to 0.8% in U.S. reformulated gasoline. Gasoline aromatics content is likely to be lowered, particularly as distillation end points (usually characterized as the 90% distillation temperature) are lowered, since the high-boiling portion of the gasoline which thereby would be eliminated usually is an aromatics concentrate. Since aromatics have been the principal source of increased gasoline octanes during the recent lead-reduction program, severe restriction of the benzene/aromatics content and high-boiling portion will present refiners with processing problems. Such problems have been addressed through such technology as isomerization of light naphtha to increase its octane number, isomerization of butanes as alkylation feedstock, and generation of additional light olefins through fluid catalytic cracking and dehydrogenation as feedstock for alkylation and production of oxygenates. This issue often has been addressed by raising the cut point between light and heavy naphtha, increasing the relative quantity of naphtha to an isomerization unit. The performance and stability of naphtha isomerization units thus are increasingly important in refinery planning.
U.S. Pat. No. 5,036,035 (Baba et al.) teaches a catalyst, and its use in isomerization, containing sulfate, zirconium oxide or hydroxide and a platinum- group metal; coverage is restricted to a catalyst consisting essentially of these components. European patent application 0,666,109 A1 discloses a sulfated catalyst, and use in isomerization, comprising an oxide or hydroxide of Group III or Group IV; oxide or hydroxide of Groups V, VI or VII; oxide or hydroxide of Group VIII; and a component from a list of Group VIII metals and metal combinations. These references disclose prereduction of the catalyst before use in isomerization, but are silent with respect to regeneration.
U.S. Pat. No. 5,675,048 (Zhang et al.) teaches a fluidized-bed alkylation process with two regeneration zones, one based upon mild hydrogenative washing for partial regeneration and one based on higher-temperature regeneration with liquid-phase hydrocarbons containing dissolved hydrogen.
The state of the art in regeneration of solid superacid catalysts is represented by U.S. Pat. No. 5,362,694 (Hollstein et al). Conventional regeneration by adding oxygen or air in increasing amounts to an inert atmosphere while heating the catalyst to 350 to 450.degree. C. is followed by introducing 1 to 20% sulfur dioxide into the stream. The procedure is said to be particularly useful for the regeneration of sulfated superacid zirconia catalysts, exemplified by activity improvement of such catalysts used in paraffin isomerization.
The problem facing workers in the art, therefore, is to find efficient and effective methods of reactivating superacid catalysts used in paraffin isomerization.